Hi Rupesh!
Thanks for reaching out so early for next year's GSoC. The best thing you can do to get started on the road to being a GSoC student in 2019 is to start working through the steps outlined at the top of this post. Learning what Plone is, how to use it as an end-user, how it works, and how to develop a website using Plone is always a great first step. The linked list of GSoC ideas is of course from this year's round of work, so many of the topics will change, but they can give you a solid idea of the types of work we like to do during GSoC.
Once again, thanks for reaching out, and welcome to the Plone community!